There are two common types of capacitive sensors. In the first type of capacitive sensor, the measurement axis is normal to the surfaces of the sensing capacitor plates. The available motion in this type of capacitive sensor is therefore limited to the gap between the capacitor plates. In the second type of capacitive sensor, the measurement axis is parallel to the surfaces of the sensing capacitor plates. Typically, the second type of capacitive sensor has much larger available motion and is used, for example, in digital vernier calipers.
Micromachined capacitive sensors have been proposed that use facing surfaces of two micromachined parts to provide position sensing. These capacitive sensors typically offer excellent position sensitivity over a large bandwidth. The moving part (rotor) is suspended by micromachined flexures above the stationary part (stator) so there is typically little friction involved in the motion. Because the capacitive sensor is micromachined, the gap between the moving part and the stationary part can be reduced in size, typically to the order of 1 μm. The small gap size typically increases the sensitivity of the capacitive sensor. The travel of the micromachined capacitive sensor is typically on the order of about 100 μm because the travel is parallel to the surfaces of the capacitor plates.
Because micromachined capacitive sensors and other micromachined type sensors are typically very fragile they are typically used to detect only their own motion and are not coupled to an external actuator which is typically a macrostructure. Micromachined accelerometers and pressure sensors have been used to sense the acceleration and pressure, respectively, of conventional macrostructures from the beginning of micromachining because the moving part does not need to be directly coupled to a macroscale object.